Preparation of photosensitive coated papers by single pass per side

ABSTRACT

PHOTOSENSITIVE PAPERS SUCH AS PROOFING PAPER, IMAGEABLE ON BOTH SIDES, ARE PRODUCED BY (A) FIRST COATING ONE SIDE WITH A COMPOSITION COMPRISING A VOLATILE ORGANIC SOLVENT AND A FILM-FORMING POLYMERIC BINDER, AND PHOTOSENSITIVE INGREDIENTS, FOLLOWED BY DRYING TO EVAPORATE THE SOLVENT AT A CONTROLLED RATE, THEREBY FORMING A FILM, THEN (B) COATING THE SECOND SIDE WITH THE SAME COMPOSITION IN A SINGLE COATING APPLICATION, AND DRYING UNDER THE COMPARABLE CONDITIONS TO THE FIRST SIDE. CAREFUL CONTROL OF THE COATING CONDITIONS, DRYING STEPS, VOLATILE ORGANIC SOLVENTS, POLYMERIC BINDERS, PHOTOSENSITIVE COMPOSITIONS AND PAPER SUBSTRATES YIELD BLISTER-FREE, TWO-SIDE COATED, PHOTOSENSITIVE PAPERS.

United States Patent 3,672,933 PREPARATION OF PHOTOSENSITIVE COATED PAPERS BY SINGLE PASS PER SIDE Marvin Frank Lieberman, Newark, Del., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del. No Drawing. Filed Nov. 30, 1970, Ser. No. 93,890 Int. Cl. B44d 1/02 US. Cl. 117-34 9 Claims ABSTRACT OF THE DISCLOSURE Photosensitive papers such as proofing paper, imageable on both sides, are produced by (a) first coating one side with a composition comprising a volatile organic solvent and a film-forming polymeric binder, and photosensitive ingredients, followed by drying to evaporate the solvent at a controlled rate, thereby forming a film, then (b) coating the second side with the same composition in a single coating application, and drying under the comparable conditions to the first side. Careful control of the coating conditions, drying steps, volatile organic solvents, polymeric binders, photosensitive compositions and paper substrates yield blister-free, two-side coated, photosensitive papers.

BACKGROUND OF THE INVENTION (1) Field of the invention This invention relates to a process for preparing blisterfree paper coated on each side by a single pass with filmforming photosensitive compositions.

(2) Description of the prior art Photosensitive formulations are conveniently applied to paper substrates as solutions in volatile organic solvents. They may be thus continuously coated on roll papers utilizing such typical devices for continuously laying down wet films as nip fed three roll reverse roll coating heads, four roll pan fed reverse roll coaters, gravure coaters, trailing blade coaters, and Mayer bar coating heads. The wet thickness is adjusted such that the dry thickness after solvent removal is in the desired range of about 0.1 to 1.5 mil, and usually around 0.3 to 0.5 mil on paper. After the solvent has been removed, such as by radiant heating or forced air evaporation, the coated paper is wound up on rolls in the usual way.

For many photoproofing applications it is highly desirable to have paper coated on both sides with the photosensitive composition. Attempts to coat both sides by the conventional manner described above, with photosensitive compositions dissolved in volatile organic solvents, have been unsuccessful. The two-side coated papers resulting have been satisfactorily coated on the first side, but the second side contained numerous blemishes, descriptively termed blisters. Blistered photosensitive paper is unacceptable to the trade primarily because of its appearance and sensitometric behavior.

Elimination of blisters has been sought by various experiments, for example, by altering drying speed, altering active ingredient content of coating formulations, and changing coating speeds. While some of these alterations give improved results, they are not entirely practicable for commercial production.

The blister problem is believed to arise from the use of a volatile organic solvent for the photosensitive compositions applied, coupled with presence of film-forming ingredients, such as polymeric binders, and the necessity to coat and dry the formulations at practical concentrations and rates. Presumably solvent from the second coating penetrates into the substrate until it reaches the first side coating which acts as a barrier. The solvent then returns to the second coating surface where it bubbles through the skin which has been formed by solvent evaporation.

It is an object of this invention to provide a method for preparing blister-free photosensitive papers coated on both sides with photosensitive compositions, with the photosensitive composition applied in a single pass to each side.

SUMMARY OF THE INVENTION 1) applying to the first side of the paper a solution,

in a volatile organic solvent, of a photosensitive formulation comprising a film-forming polymeric binder and a photosensitive component in an amount sufficient to provide a dry film coating of about 3 to 15 pounds per 3000 square feet of paper;

(2) drying the first-side coat by evaporating the solvent at a rate effective to yield a dry, blister-free film, then .(3) applying to the second side of the paper the solution of step (1) in a single application and in an amount sufficient to provide a dry film of a weight of about 3 to 15 pounds per 3000 square feet of paper and sufiicient to give a dry blister-free film, and drying the resultant coat by evaporating the solvent at a rate effective to yield a dry, blister-free film, the holdout value to coating weight ratio being at least about 2:1.

DESCRIPTION OF THE INVENTION This invention is directed to a process for applying photosensitive formulations, comprised principally of a volatile organic solvent, a photosensitive component and a polymeric binder, to both sides of paper which is readily permeable by said solvent and less than 10 mils thick. The inventive process is effective with conventional coating and drying machinery, in spite of the fact that the first side coat is sufiiciently thick to be substantially impenetrable, or only slowly penetrable, by said solvent. The second-side coat is applied in a single step in an amount which yields a dry blister-lfree film and based on the holdout value of the paper, and is dried under controlled drying conditions.

A solution to the problem of obtaining blister-free, twoside coated, photosensitive papers depends on the use of paper which is relatively impenetrable by the volatile organic solvent used to apply the coatings. Solvent penetrability of paper can be measured, and is conveniently termed solvent holdout. Thus, papers useful in this embodiment of the invention are papers, 1-10 mil, preferably 25 mil thick, and exhibiting a solvent holdout of greater than 5, preferably at least 75, and most peferably 170.

In describing the volatile organic solvent holdout characteristics of the paper, it should be pointed out that the solvent holdout required is related to the thickness of the second-side coat applied because the inside (first) coating applied is relatively solvent impenetrable. Thus, solvent applied with the second coating cannot readily permeate out through that side, and since the paper is relatively thin (1 to 10 mil), it is too thin to serve as a sink or trap for the solvent.

On applying the second coat, which initially contains a volatile organic solvent as major component, this solvent must be removed to deposit a smooth film of binderphotosensitive components. If only a thin film is applied as second coat, there is a relatively small quantity of solvent present to be removed, or to penetrate through the paper, strike the solvent impermeable first coat, and return through the second coat to possibly form blisters. Hence, with thin second coats, papers with lower solvent holdout will suflice. On the other hand, when heavy second coats are applied, relatively large quantities of solvent are present, which must be prevented from returning through the second coat, to form blisters. Thus, with heavy second coats, paper with high solvent holdout is necessary.

A relationship between holdout value and coating weights, with the preferred photosensitive compositions applied in this invention, can be seen from the examples. As evidenced by the data in Tables I and II of the examples, these ratios may vary somewhat with coating and drying conditions; however, the relationship is obvious.

Thus, when intermediate low weights (4-6 lbs.) are to be applied to the second side, a ratio of second-side organic sol-vent holdout value to second-side coating weight of as low as 2/1 can be adequate with careful application (holdout value as low as 6-10) while a ratio of approximately 16/1 is preferred (holdout value of at least 75). On the other hand, when higher coating weights (7-12 lbs.) are to be applied single pass per side, as is preferred, a ratio of 23/1 or higher is preferable (holdout value of at least 170).

(l) PAPER SUBSTRATE Since the principal object of this invention is to prepare blister-free, two-side coated, photosensitive papers, it should first be noted that the paper used must be essentially opaque to ultraviolet and visible light. This is critical for two-side coated photosensitive papers.

It is also important that the volatile organic solvent permeability of the paper to be coated be or more, preferably higher than 75, and most preferably higher than 170. It is also important that the paper be thin. Thick paper (thicker than 10 mil) can be more easily coated on both sides without encountering a blistering problem. The papers of this invention are 1 to 10 mils thick and preferably 2 to 5 mils thick.

Resistance to solvent penetration, or solvent holdout as used herein, is measured by the Schweitzer test which defines the holdout as the time required for a dye solution to penetrate through a sheet of the paper being tested.

The Schweitzer test measures the time, in seconds, necessary for a standard dye solution to sufficiently penetrate through a sheet of paper to reduce the reflectance value from the paper by percent. To be more specific, a sheet of paper is laid horizontally over the aperture leading to photocells and apparatus for measuring reflectance. The dye solution is applied to the top of the paper; on penetrating the paper, the dye eventually reaches the back-side, the side exposed to the reflectometer. The dye absorbs light and thus reduces reflectance. Arbitrarily, a 20 percent reduction in reflectance is chosen for convenience in measurement. The standard dye solution contains 0.2 gram Naphthol Green B (Colour Index 10020, Acid Green 1) dissolved in ml. water and 100 ml. 95 percent ethanol.

Alternative methods for determining paper permeability may be used. One method which measures paper porosity is defined by TAPPI test method T-460 05-68 for determining the air resistance of paper; the test results are usually expressed as a Gurley number.

The higher the Gurley number of the paper, i.e., the more impervious the paper to air, the higher the secondside coating weight that can be applied blister-free in a single pass. Papers with a Gurley value greater than 400 are particularly useful for applying full coating weights, i.e. 8 to 10 lbs. per 3000 square feet single pass per side to yield blister-free two-sided coated products.

It is very desirable, with the formulations described herein, to apply the coatings to both sides of paper in the range of 5 to 15 pounds and preferably 7 to 12 pounds per 3000 square feet. At this higher weight many commercial papers would yield a blistered product when applied single pass per side. The common commercial papers which exhibit this defect include 25 pounds per, 32 pounds per and 42 pounds per 2000 square feet regular HG papers of PJ. Schweitzer Division, Kimberly Clark Corp. and their 32 pounds per 2000 square feet grades HG-XH and HG-XL.

(2) SOLVENTS Suitable solvents are those inert toward the photo sensitive composition, for example a leuco dye or salt thereof, a photo-oxidant, and a binder. The solvents are conveniently employed to dissolve these components, mix them, and provide fluid homogeneous media for application to substrates. In general, a solvent should dissolve all components, but be inert to them. It should also be inert to and wet, but not dissolve, the paper to which the photosensitive materials are applied.

To obtain the final coating, the solvent is removed by evaporation. Removal should be sufiicient to yield a coating dry to the touch and the term dry as used herein will be used to mean dry to the touch.

Volatile organic solvents suitable for use in this invention include those boiling in the range 40-120 C. Representative of such solvents are methanol, ethanol, acetone, l-propanol, 2-propanol, n-butanol, methyl acetate, ethyl acetate, benzene, toluene, methyl ethyl ketone, 3-pentanone, methylene chloride, chloroform, 1,1,2-trichloroethane, tetrahydrofuran, dioxane, and mixtures thereof in various proportions as may be required to dissolve the various components selected for use in the composition. It is particularly desirable that the solvents be volatile enough to be readily removed with low expenditure of thermal energy. Preferred solvents are those boiling between 55 and 85 C.

The volatile organic solvent is the major ingredient of the photosensitive formulations applied by the process of this invention. Usually, the volatile organic solvent comprises 65-90% by weight of the photosensitive formulation to be applied to the paper surface. Preferably the solvent comprises 85% by weight of the formulation.

(3) BINDERS Polymeric binders are present in light-sensitive compositions to thicken and adhere them to the paper substrate and to serve as a matrix for the color-forming photosensitive components. Light-transparent and film-forming polymers are preferred. Representative of suitable polymers are ethyl cellulose, polyvinyl alcohol, polyvinyl chloride, polystyrene, polyvinyl acetate, poly(methyl methacrylate), cellulose acetate, cellulose butyrate, cellulose acetate butyrate, cellulose nitrate, chlorinated rubber, copolymers of the above vinyl monomers, and gelatin. Binder or matrix amounts vary from about 1 part to about 200 parts, preferably 3 to 15 parts, by weight per part of combined weight of the photosensitive components.

When two-side coating some of the preferred compositions exemplified hereinafter to a dry coating weight of from 7 to 12 pounds per 3000 square feet, the percentage of binder in the initially applied solution is normally 8 to 15 percent by weight.

(4 PLASTICIZER With certain compositions it may be desirable to add a plasticizer to give flexibility to the film or coating. Plasticizers include the polyethylene glycols such as the commercially available Carbowaxes, and related materials such as substituted phenol-ethylene oxide adducts, for example the polyethers obtained from o-, mand p-cresol, o-, mand p-phenylphenol and p-nonylphenol, including commercially available materials such as the Igepal alkyl phenoxy polyoxyethylene ethanols. Other suitable plasticizers are the acetates, propionates, butyrates and other carboxylate esters of ethylene glycol, diethylene glycol, glycerol, pentaerythritol and other polyhydric alcohols, and alkyl phthalates and phosphates such as dimethyl phthalate, diethyl phthalate, dioctyl, phthalate, tributyl phosphate, trihexyl phosphate, trioctyl phosphate, triphenyl phosphate, tricresyl phosphate and cresyl diphenyl phosphate. The quantity of optionally added plasticizer is usually not more than by weight, of the binder present.

(5) PHOTOSENSITIVE COMPONENTS The other ingredients of the photosensitive formulations are known in the art. These components are not critical and may vary widely provided they are soluble in the solvent, are non-volatile, and do not interfere with the coating operation. Some preferred photosensitive compositions are described in US. Pat. 3,445,234, US. Pat. 3,390,996, US. Pat. 3,390,994, and application Ser. No. 99,512, filed Dec. 18, 1970. Other suitable photosensitive compositions are disclosed in US. Pat. 3,585,038, and US. Pat. 3,598,992.

(6) COATING METHODS (A) General Conventional, continuous coating apparatus and technology are used in this invention. One method in current use is a continuous one in which paper or film from a storage roll is continuously passed by a coating head which applies the formulation, passed through a tunnel dryer in which hot air impingement is used to vaporize the solvent, and finally is passed to storage on a collecting roll.

Suitable conventional methods include gravure, Mayer bar, reverse roll and knife-over-roll, etc.

(B) Specific Photosensitive formulations of from 10 to 40 percent solids are applied to the first side of paper with a coating machine such as, but not limited to, 3 or 4 roll reverse roll, Mayer bar, or blade coater at web speeds of from 4 to 1500 feet per minute. The dry weight of the coating can range from about 3 to pounds, preferably from 5 to 15 pounds and most preferably from 7 to 12 pounds per 3000 square feet.

The first-side coating is then dried, for example, in a hot air impingement dryer. Other types of drying such as radiant heat, microwave, and others can also be used.

After the coating has been applied and dried on the first side, the web is reversed and a coating is applied to the second side. The second side coating must be dried carefully and gently to avoid blistering.

The coating weight that can be applied in one pass depends largely on the solvent hold-out characteristics of the substrate used. The poorer the solvent hold-out, the lighter the coating weight per pass. The coating weight that can be applied is also directly proportional to the percent solids in the coating lacquer. As an example, for Schweitzer 32 pound HG paper which has fairly low solvent hold-out (approximately 6-7), using the formulation of Example 7 of Belgian Pat. 716,037 and at 17 to 18 percent solids, the maximum coating that can be applied in a single first pass on the second side is in the range of 3 or 4 pounds per 3000 square feet, while at 20 percent solids the maximum coating is in the range of 5 to 6 pounds.

While this invention has been described primarily as one directed to two-side coatings of photosensitive formulations, as will be apparent to those skilled in the art, the process of this invention is equally applicable to the preparation of coated substrates in which only the second side is coated with a film-forming photosensitive composition, or in which neither side is coatedwith such a composition. This is true whenever the first side coating is one which is substantially impermeable to the solvent employed in the second side coating, the second side coating forms a film on drying, and the substrate has the solvent holdout characteristics as defined herein. Under these circumstances, when the substrate is paper, this embodiment of the invention is a blister-free paper comprismg (a) Paper less than 10 mil thick, having a solvent holdout value of 5 or more; and

(b) A coating on one side of a material in the form of a film which is not readily penetrated by volatile organic solvents; and

(c) A coating on the second side of a formulation comprising a solvent and a film-forming component, said coating being in the form of a film not readily penetrated by volatile organic solvents, said film being present in an amount ranging from about 3 to 15 pounds per 3000 square feet of paper, and said coating being applied by (l) applying to the paper in a single application a solution, in a volatile organic solvent, of a formulation comprising a film-forming component in an amount sufficient to provide a dry film coating of about 3 to 15 pounds per 3000 square feet of paper and sufiicient to give a ratio of holdout valuezfilm weight of greater than 12:1; and

(2) drying the resultant coating by evaporating the solvent at a rate effective to yield a dry, blister-free film.

Representative of materials which may be applied by this process are coatings of magnetic oxides, various antiblocking agents, anti-static agents, and other paper coating materials known to the art which form a film not readily penetrated by volatile organic solvents.

The following examples illustrate this invention in greater detail.

EXAMPLE 1 A typical photosensitive composition was prepared containing the following ingredients:

Grams 2,2 bis(o chlorophenyl) 4,4',5,5 tetrakis (m-methoxyphenyl)biimidazole 0.4180

Tris(4-diethylaminoo-tolyl)methane 0.0900 p-Toluenesulfonic acid monohydrate 0.400 Trirnethyl 3,3',3"-nitrilotripropionate 1.0 mil. 9,10-phenanthrenequinone 0.054 Cellulose acetate butyrate (Eastman Chemicals EAB-171-40) 6.0 Polyethyleneoxide adduct of o-phenylphenol 2.25 moles of ethylene oxide per mole of the phenol, average formula The above formulation as a 20 percent solids solution in :10 acetonez2-propanol mixture was coated on the papers in Table I. The papers were 20 inches wide and the felt side was coated first to yield a dry coating weight of 9.6 pounds per 3000 square feet. Drying by solvent evaporation was effected in a two-zone dryer at temperatures of F. in zone 1 and F. in zone 2.

The wire side of the paper was then coated to yield the dry weight indicated in the table, and the coatings were dried in the manner as described above. In this manner, blister-free, two-side coated photosensitive papers were obtained. Heavier second side coatings produced blistered papers.

TABLE I Coatin weight, Holdout value lbs/3,000 It. 2

Paper, First Second First Second 321b./2,000ft. B side side side side Ratio Value given by P. J. Schweitzer Paper 00. Second side holdout value/second side coating weight.

EXAMPLE 2 Table II shows the results obtained on coating the formulation of Example 1 on both sides of the paper, applied single side per pass, at a convenient coating weight for the photosensitive compositions described, namely 10 pounds per 3000 square feet, under slightly different conditions than in Example 1.

*Holdout value provided by P. J. Schweitzer Paper 00. "2nd side holdout value/2nd side coating weight.

EXAMPLE 3 The formulation of Example 1 was applied at a concentration of percent solids by the procedure of Example 1 to Schweitzer HG paper (32 pounds per 2000 square feet) having a first side holdout value of about 7 and a second side holdout value of about 6. The felt side coating was applied at a dry weight of 9.6 pounds per 3000 square feet and dried at 150 F. in zone 1 and 275 F. in zone 2.

The wire side was then coated to yield a dry weight of about 5 pounds per 3000 square feet and the second side was dried as the first to give a blister-free paper.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of applying blister-free, photosensitive coating to both sides of paper 1 to 10 mil thick comprising the steps of (1) applying to the first side of the paper a solution,

in a volatile organic solvent having a boiling point between and 120 C., of a photosensitive formulation comprising a film-forming polymeric binder which on drying forms a film substantially impenetrable by the solvent, and a photosensitive component, in an amount sufiicient to provide a dry film coating of about 3 to 15 pounds per 3000 square feet of paper and a ratio of Schweitzer solvent holdout value to dry coating weight of at least 2: 1;

(2) drying the first side coat by evaporating the solvent at a rate effective to yield a dry, blister-free film; and

(3) applying to the second side of the paper the solution of step (1) in a single application and in an amount suflicient to provide a dry film of a weight of about 3 to 15 pounds per 3000 square feet of paper and sufiicient to give a dry blister-free film, and drying the resultant coat by evaporating the solvent at a rate effective to yield a dry, blister-free film.

2. The method of claim 1 wherein the ratio of holdout value to film weight is greater than 12:1.

3. The method of claim 1 wherein the paper is 2 to 5 mils thick.

4. The method of claim 1 wherein the solvent holdout value of the paper is at least 75, the amount of coating applied to each side is sufiicient to give a dry coating weight in the range of 5 to 15 pounds per 3000 square feet of paper, and the ratio of holdout value to dry coating weight is at least 16: 1.

5. The method of claim 4 wherein the solvent has a boiling point between 55 C. and 85 C.

6. The method of claim 1 wherein the solvent holdout value of the paper is at least 170, the amount of coating applied to each side is suflicient to give a dry coating weight in the range of 7 to 12 pounds per 3000 square feet of paper, and the ratio of holdout value to dry coating weight is at least 25:1.

7. Blister-free photosensitive paper comprising (a) paper essentially opaque to actinic radiation, 2 to 5 mils thick, having a solvent holdout value of at least 75, and a coating on both sides of (b) a photosensitive formulation comprising a filmforming polymeric binder and a photosensitive component, said coating being in the form of a film not readily penetrated by volatile organic solvents, said film being present on each side of the paper in an amount ranging from 5 to 15 pounds per 3000 square feet of paper; and

said coating being applied by the process of claim 1.

8. A method of applying a blister-free photosensitive coating to the second side of paper 1 to 10 mil thick, said paper having a solvent holdout of at least 5, and a coating on the first side of a filrn not readily penetrated by volatile organic solvents, said method comprising the steps of (a) applying to the second side of the paper in a single application a solution, in a volatile organic solvent, of a photosensitive formulation comprising a filmforming polymeric binder and a photosensitive component, in an amount suflicient to provide a dry film coating of about 3 to 15 pounds per 3000 square feet of paper and sufficient to give a ratio of holdout value: film weight of greater than 12:1; and

(b) drying the resultant coating by evaporating the solvent at a rate effective to yield a dry, blister-free film.

9. Blister-free photosensitive paper comprising (a) paper less than 10 mil thick, having a solvent holdout value of at least and (b) a coating on one side of a material in the form of a film which is not readily penetrated by volatile organic solvents; and

(c) a coating on the second side of a photosensitive formulation comprising a film-forming polymeric binder and a photosensitive component, said coating being in the form of a film not readily penetrated by volatile organic solvents, said film being present in an amount ranging from about 3 to 15 pounds per 3000 square feet of paper; and

said coating being applied by the process of claim 8.

References Cited UNITED STATES PATENTS 3,196,038 7/1965 Schoch et a1. 1l768 2,993,803 '7/1961 Sulich et al. 9675 2,889,806 6/1959 Conant 1l768 NORMAN G. TORCHIN, Primary Examiner J. WINKEIJMAN, Assistant Examiner U.S. c1. X.R. 1l768, 152, 15s 

